Just like you're right or left handed you also have a dominent eye. In most cases this is the eye that prefers visual input and the one you should use when looking through a viewfinder and taking photos (or shooting, darts, and any other one-eye activity). Reddit user ElementK shares an easy way to determine your dominent eye.

ElementK's technique is one of many but it's one of the easiest to do:

Extend both hands forward of your body and place the hands together making a small triangle (approximately 1/2 to 3/4 inch per side) between your thumbs and the first knuckle like this.

With both eyes open, look through the triangle and center something such as a doorknob in the triangle.

Close your left eye. If the object remains in view, you are right eye dominant. If closing your right eye keeps the object in view, you are left eye dominant.

Whichever eye is dominent should be used to look through the viewfinder of your camera. Give it a shot for yourself, you might be surprised with the results.